The invention relates to a method for communicating by radio, whereby a frequency band divided up into a plurality of subcarriers is used for the communication. The invention also relates to a transmitter and a receiver for implementing the method.
In radio communication systems, messages containing for example voice information, image information, video information, SMS (Short Message Service), MMS (Multimedia Messaging Service) or other data are transmitted between transmitter and receiver by way of a radio interface with the aid of electromagnetic waves. With regard to these radio stations, depending on the actual embodiment of the radio communication system these can be different types of user stations or network-side base stations. In a mobile radio communication system, at least some of the user stations are mobile radio stations. The emission of the electromagnetic waves takes place using carrier frequencies which lie in the frequency band provided for the system in question.
Mobile radio communication systems are often implemented as cellular systems, for example according to the GSM standard (Global System for Mobile Communication) or UMTS standard (Universal Mobile Telecommunications System) with a network infrastructure comprising for example base stations, facilities for checking and controlling the base stations and additional network-side facilities. Apart from these (supralocal) cellular, hierarchical radio networks organized on a wide scale, there are also wireless local networks (WLANs, Wireless Local Area Networks) which have a radio coverage area that is as a rule significantly more restricted in spatial terms. Examples of different standards for WLANs are HiperLAN, DECT IEEE 802.11, Bluetooth and WATM.
Access by user stations to the common transmission medium is controlled with regard to radio communication systems by multiple access methods/multiplexing methods (Multiple Access, MA). In the case of these multiple accesses, the transmission medium can be divided up between the user stations in the time range (Time Division Multiple Access, TDMA), in the frequency range (Frequency Division Multiple Access, FDMA), in the code range (Code Division Multiple Access, CDMA) or in the space range (Space Division Multiple Access, SDMA). Combinations of multiple access methods are also possible, such as for example the combination of a frequency range multiple access method with a code range multiple access method.
In order to achieve the most efficient possible transmission of data, the entire available frequency band is split up into a plurality of subcarriers (multicarrier method). The idea behind these FDMA methods is to convert the initial problem of transmitting a wideband signal into the transmission of a plurality of narrowband signals. Among other things, this has the advantage that the complexity required in the receiver can be reduced. In addition, the division of the available bandwidth into a plurality of narrowband subcarriers permits a significantly higher granularity of the radio resources, in other words the radio resources can be distributed over the data to be transmitted or over the user stations with a high level of refinement. In the case of transmissions having a variable data rate or in the case of burst-mode data traffic in particular, the available bandwidth can be efficiently utilized by allocating subcarriers to different user stations.
One example of a multicarrier transmission method is the OFDM (Orthogonal Frequency Division Multiplexing) system in which temporally approximately squarewave pulse shapes are used for the subcarriers. The frequency spacing of the subcarriers is chosen such that in the frequency range at the particular frequency at which the signal of a subcarrier is evaluated the signals of the other subcarriers exhibit a zero passage. The subcarriers are thus orthogonal with respect to one another. As a result of the generally extremely small spacing of the subcarriers and the narrowband nature of the signals transmitted onto the individual subcarrier in the case of OFDM, it is necessary to ensure that the transmission within the individual subcarriers is generally not frequency selective. This facilitates signal equalization at the receiver.